1. Field of the Invention
The invention relates to a magnetic-tape guide arrangement for a transversely reading and/or writing machine for magnetically recording and/or reproducing signals of large bandwidth, for example video signals, on a magnetic tape which is moved in its longitudinal direction, and hereinafter referred to as a transverse scan tape recorder; and more particularly to such a tape recorder having a multiplicity of magnetic heads which are mounted on a head disc rotatable about an axis of rotation which is parallel to the direction of movement of the magnetic tape, and comprises a pressure member which is transversely (radially) movable relative to said axis of rotation.
Such a recorder also includes two at least partly cylindrical drums which are coaxial with said axis of rotation and which are disposed on either side of the head disc, as well as resilient means for pressing the pressure member against the drums in the operating condition of the machine, the pressure member having concave guide portions and each of the two drums having convex guide portions for guiding the magnetic tape in a locally cupped condition along a part of the circular path of the magnetic heads. The axial distance between the convex guide portions in the direction of the axis of rotation of the head disc is greater than the axial distance between the concave guide portions, and the guide portions are so arranged relative to each other that in the operating condition a gap is formed between each concave guide portion and the nearest convex guide portion where the magnetic tape is entirely clear of the guide portions, while the pressure member and the drums contact each other at the location of the concave and convex supporting portions of substantially equal radius of curvature.
2. Description of the Prior Art
Such a magnetic-tape guide arrangement is described in the Applicant's co-pending Patent Application Ser. No. 636,925, filed Dec. 2, 1975, now U.S. Pat. No. 4,012,787. The magnetic-tape guide arrangement described therein forms part of a transversely scanning recorder of the type described above. The two drums between which the head disc is rotatably disposed are mounted on a mounting block which is provided with a V-shaped groove which cooperates with cylindrical portions of the two drums, so that an excellent alignment and positioning of the drums is obtained. The mounting block and the pressure member are radially movable relative to each other, so that it is possible to feed the tape between the pressure member and the drums in stretched form, before it is locally cupped by moving the drum and the pressure member towards each other. The two drums are disposed comparatively close to each other and the pressure member has such a width that it partly bridges the gap between the two drums. In the pressure member a cylindrical recess is formed whose radius corresponds to that of the two drums and the entire width of the pressure member functions as concave guide portion, except for a recessed part at the location of the center of the pressure member. This recessed part corresponds to the location where the video heads mounted on the head disc are in contact with the magnetic tape. During operation these heads will deform the magnetic tape over a slight depth. The two drums have recesses which extend axially beyond the width of the pressure member. Thus it is ensured that the magnetic tape between each convex and concave guide portion bridges a gap where the magnetic tape is entirely clear of any machine part.
The said recesses in the drums cover an arc along the cylindrical surface of the drums which is slightly greater than the arc which is covered by the cupped magnetic tape. With its cylindrical recess the pressure member covers an arc which is greater than the arc covered by the recesses in the cylindrical surface of the drums, in such a way that on both sides of the magnetic tape a portion of the pressure member extends which serves as a convex supporting portion. These two supporting portions on either side of the magnetic tape cooperate with corresponding portion of the two drums. These last-mentioned portions consequently serve as concave supporting portions.
Thus, in the known magnetic-tape guide arrangement the pressure member is supported at the top and bottom of the magnetic tape, while the magnetic tape is guided by the two drums at the left and right on both sides of the pressure member. This means that the axial distance between the cylindrical portions of the drums at the top and bottom of the tape should be smaller than the axial distance between the cylindrical portions of the drums which function as convex guide portions. In the known magnetic tape guide arrangement the distance at the said location is just great enough to leave the head disc with its heads free.
The known magnetic-tape guide arrangement has two drawbacks. First of all the stability of the pressure member support on the two drum sections is inadequate because of the limited width of the pressure member. A second drawback is a fact that the recesses must be formed in the two drums resulting in a transition from the cylindrical surface of the drum to the surface of the recess at the location where the pressure member bears on the drums. Forming such a recess demands additional operations, while at the transition of said two surfaces local deformations may be produced which tend to mis-align the pressure member.